1. Field of the Invention
The present invention relates to methods for determining and controlling the quality of optical properties of base material used in the fabrication of semi-finished ophthalmic lenses.
2. Description of the Related Art
In the ophthalmic lens manufacturing industry, polycarbonate material used in the fabrication of lenses is selected and purchased by manufacturers based upon testing of chipper plate samples representative of a production lot of the polycarbonate material. The production lot of polycarbonate material is commonly produced in the form of pellets having a length of between two and four millimeters and a diameter of between one and four millimeters. Typically, lens manufactures select candidate materials from a supplier catalogue describing optical property specifications. Although the general chemical composition of various production lots representing a specific material is essentially the same, different production lots of the material used in the fabrication of lenses commonly result in lenses that have different optical qualities, such as color and ultra-violet light transmission, due to variations in manufacturing conditions of the production lot. Such variations may include different raw material characteristics, concentration variations, production machine differences, feed rate inaccuracies in processing, environmental impurities, and other factors. Accordingly, to enable lens manufacturers and other customers to confirm the optical properties of a specific production lot, suppliers provide customers with a rectangular, five-millimeter thick, standard chipper plate with a dimension of approximately a 50 mm×75 mm molded from the production lot to permit customers to conduct quality testing. Upon confirming that the chipper plate meets the customer's specification requirements, the production lot is purchased from the supplier and shipped to the customer. Depending on the production scale of particular lens manufacturers, a production lot size is commonly between several hundred pounds to several tons.
While the standard chipper plate employed by suppliers may be sufficient to provide an indication of optical properties of a polycarbonate production lot for many applications, the standard chipper plate is typically a poor indicator of optical properties of semi-finished ophthalmic lenses fabricated from the production lot. Lens manufacturer color, ultra-violet light transmission and other optical property specifications are especially precise and the current method by which lens manufacturers obtain an indication of color properties is inadequate in providing a true measure of color properties of a semi-finished lens molded from the production lot. For example, a high level diagram of a prior art production lot confirmation and purchase process 100 is provided in FIG. 1A. The process may begin at step 102 in which a 5-mm thick standard chipper plate is provided to a customer for quality testing. If the initial testing reveals that the production lot meets customer requirements, the customer may purchase and receive production lot pellets at step 104. Thereafter, at step 106, lenses are fabricated from the production lot and, at step 108, an accurate measure of product color, for example, is obtained by quality control testing. In most cases, the lens color properties are very different from the measured color properties of the 5-mm thick standard chipper plate.
To improve the results of the initial testing of the standard chipper plate, and thereby avoid purchasing a defective production lot, lens manufactures have resorted to mathematical extrapolation schemes based on historical data to predict a lens color of lenses molded from the production lot. FIG. 2A is a c*h* scale color plot 200 that provides an illustrative example of mathematical extrapolation employed by lens manufacturers. Element 202 is an exemplary representation of color values measured from a standard chipper plate and element 209 is an acceptable color region of molded articles suitable for use in ophthalmic lenses. Angular vectors 206 are determined based on historical data to extrapolate the color value 202 and to construct a color region 208 indicating a prediction of color properties that semi-finished lenses produced from a candidate production lot would possess. A lens manufacturer then chooses to either accept or reject the production lot based on any overlap between region 208 and 209.
Referring to FIG. 1B with continuing reference to FIG. 2A, process 120 provides an illustrative example of a production lot quality assessment method utilizing mathematical extrapolation. For example, at step 122, a 5-mm standard chipper plate representative of a production lot is provided to the customer, who in turn performs color measurements to obtain color values 202. At step 124, extrapolation is performed as discussed above and a predicted color region 208 is obtained. If the predicted region does not indicate that an ophthalmic lens fabricated from the production lot would have color properties within acceptable region 209, then the production lot is rejected. Otherwise, the optical properties are confirmed and a customer may request and receive production lot pellets at step 126. At step 128, the production lot is employed to mold semi-finished lenses at a mass production scale. Thereafter, at step 130, quality control testing may be performed on the lenses to determine their true corresponding color properties and assess whether the customer's specification requirements are met.
Despite the performance of mathematical extrapolation, the resulting data is not guaranteed to provide color qualities that accurately indicate the color properties of semi-finished lenses manufactured from the production lot. Thus, even if color specifications are met in initial testing, color deficiencies may be revealed during a lens mass production process due to variations in manufacturing conditions after a production lot size of several hundred pounds has been purchased. Conversely, because of the uncertainty of the predicted color region determined through mathematical extrapolation, there are situations in which a production lot may meet a customer's requirements but has been rejected because of a small overlap between possible predicted color values 208 and a color region 209 in accordance with a customer's color specifications.
Accordingly, there is a need for an accurate and efficient quality assessment method for a production lot of polycarbonate material used in the fabrication of ophthalmic lenses that does not require generation of semi-finished lenses or mathematical extrapolation schemes. In addition, there is also a need for enhanced chipper plates that provide an accurate measure of optical properties of ophthalmic lenses generated from a production lot to enable a both suppliers and customers to determine whether the production lot meets optical quality specifications.